Window watchdog concept in Embedded C - Step-by-Step Execution

Concept Flow - Window watchdog concept

Start Program

↓

Initialize Window Watchdog

↓

Enter Main Loop

↓

Check if Refresh Window Open?

No→System Reset

Yes↓

Refresh Watchdog Counter

↓

Continue Normal Operation

↩Back to Main Loop

The program initializes the watchdog, then repeatedly checks if the refresh happens within the allowed time window. If refreshed too early or too late, the system resets.

Execution Sample

Embedded C

WWDG_Init();
while(1) {
  if (within_refresh_window()) {
    WWDG_Refresh();
  } else {
    // do not refresh
  }
}

This code initializes the window watchdog and refreshes it only when inside the allowed time window to prevent system reset.

Execution Table

Step	Watchdog Counter	Time	Condition: Inside Window?	Action	System State
1	Start at max (e.g. 127)	0 ms	N/A	Initialize watchdog	Running
2	126	10 ms	No (too early)	No refresh	Running
3	125	20 ms	Yes (inside window)	Refresh watchdog	Running
4	127	21 ms	No (too early after refresh)	No refresh	Running
5	125	30 ms	Yes (inside window)	Refresh watchdog	Running
6	127	31 ms	No (too early)	No refresh	Running
7	126	40 ms	No (too late, missed window)	No refresh	System Reset
Exit	-	-	-	-	System reset triggered due to missed refresh window

💡 System resets because the watchdog was not refreshed within the allowed time window.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	Final
Watchdog Counter	127	126	127	126	127	126	126	-
System State	Running	Running	Running	Running	Running	Running	System Reset	System Reset

Key Moments - 2 Insights

Why can't we refresh the watchdog counter too early?

Refreshing too early means outside the allowed window, so the watchdog treats it as a fault and will reset the system, as shown in steps 2 and 4 where no refresh happens because it's too early.

What happens if we miss the refresh window completely?

If the refresh window is missed, the watchdog counter reaches a critical value and triggers a system reset, as seen in step 7 where the system resets due to no refresh.

Visual Quiz - 3 Questions

Look at the execution table, what is the watchdog counter value at step 3 after refresh?

A 126

B 125

C 127

D 124

💡 Hint

Step 3 shows 125 before refresh, but after refresh reloads to max 127 (as step 4 starts at 127).

At which step does the system reset occur according to the execution table?

A Step 6

B Step 7

C Step 5

D Step 4

💡 Hint

Look at the 'System State' column for the first occurrence of 'System Reset'.

If the watchdog is refreshed too early at step 2, what would happen?

A System reset triggered immediately

B Watchdog counter increases

C System continues running normally

D Watchdog counter freezes

💡 Hint

Refer to the explanation in key_moments about refreshing too early causing reset.

Concept Snapshot

Window Watchdog Concept:
- Initializes with a counter that counts down.
- Must refresh only within a specific time window.
- Refreshing too early or too late causes system reset.
- Helps detect software faults by enforcing timing constraints.
- Used in embedded systems for safety and reliability.

Full Transcript

This visual execution trace shows how a window watchdog timer works in embedded C. The watchdog counter starts at a maximum value and counts down over time. The program must refresh the watchdog only when inside a specific time window. Refreshing too early or too late causes the watchdog to reset the system. The execution table shows each step with the watchdog counter value, time, condition check, action taken, and system state. Variable tracking shows how the watchdog counter and system state change over time. Key moments clarify why refreshing too early or missing the window causes reset. The quiz questions test understanding of the watchdog counter values and reset timing. This concept helps embedded programmers ensure their system recovers from software faults by enforcing timely refreshes.